Method and apparatus for spark spectroscopy by deriving light from limited portions of the spark discharge

ABSTRACT

A stabilized spark discharge is produced so that the electrical spark always follows substantially the same path or channel between anode and cathode electrodes forming an analytical spark gap. The cathode is preferably made of, coated with, or otherwise supplied with the sample material to be analyzed. Greatly improved spectroscopic analysis is made possible be deriving the light for carring out such spectroscopic analysis only from preselected portions of the spark discharge such as the luminous wings outside the spark channel. Such unique portions or shells are caused by the rapid expansion of the plume or cloud of the sample material explosively vaporized from the cathode by the spark discharge. The spectral lines derived from the wings of the discharge are due largely to the excited vapor of the sample material without substantial interference from the complicated background spectral lines emitted by the spark channel due to the ionized gas which supports the spark discharge and also due to the interaction between the ionized gas and the vapor of the sample material. The spark discharge may be stabilized by producing the discharge between an anode in the form of a sharply pointed pin and cathode having a flat or other surface confronting the anode; and by surrounding the sharply pointed pin with a laminar stream of argon or some otjer readily ionizable gas directed along the axis of the spark gas so as to impinge upon the cathode, and flowing at a rapid rate so as to sweep away or spatially arrange the products of each spark between the repetitive sparks. The spark current should be uni-directional to enhance the discharge stability and to avoid the vaporization of solid material from the anode.

United States Patent 1191 Walterset al. g

[ June 11, 1974 FROM LIMITED PORTIONS OF THE SPARK DISCHARGE [75] Inventors: John P. Walters; Steven A.

Goldstein; William S. Eaton, all of Madison, Wis.

[73] Assignee: Wisconsin Alumni Research 1 Foundation, Madison, Wis.

[22 Filed: Feb. 4, 1972 [211 App]. No.: 223,605

[52] US. Cl 356/86, 356/36 [51] Int. Cl. G0lj 3/30 [58] Field of Search 356/86, 36, 30, 31; 250/237 G [56] References Cited 5 UNITED STATES PATENTS 1,918,976 7/1933 Marrison 356/31 2.367.704 l/1945 Walker 356/36 2.744.438 5/1956 Steinhaus et al.. 356/86 2.758.238 8/1956 Todd .L 356/86 3.578.980 6/1969 Decker. Jr. 250/237 G 3,583.8!2 6/l97l Blum et al. 356/86 3.653.766 4/1972 Walters et al 356/86 3.669.546 6/1972 Virloget 356/86 OTHER PUBLICATIONS Emission Spectrometry, Margoshes et al.; Analytical Chemistry; Vol. 42, No. 5; April 1970.

Primary ExaminerRonald L. Wibert Assistant Examiner-V. P. McGraw SITE.

[5 7] ABSTRACT A stabilized spark discharge is produced so that the electrical spark always follows substantially the same path .or channel between anode and cathode electrodes forming an analytical spark gap. The cathode is preferably made of, coated with, or otherwise supplied with the sample material to be analyzed. Greatly improved spectroscopic analysis is made possible be deriving the light. for earring out such spectroscopic analysis only from preselected portions of the spark discharge such as the luminous wings outside the spark channel. Such unique portions or shells are caused by the rapid expansion of the plume or cloud of the sample material explosively vaporized from the cathode by the spark discharge. The spectral lines derived from the wings of the discharge are due largely to the excited vapor of the sample material without substantial interference from the complicated back ground spectral lines emitted by the spark channel due to the ionized gas which supports the spark discharge and also due to the interaction between the ionized gas and the vapor of the sample material. The spark discharge may be stabilized by producing the discharge between an anode in the form of a sharply pointed pin and cathode having a flat or other surface confronting the anode; and by surrounding the sharply pointed pin with a laminar stream of argon or some otjer readily ionizable gas directed along the axis of 1 the spark gas so as to impinge upon the cathode. and

flowing at a rapid rate so as to sweep away or spatially arrange the productsof each spark between the repetitive sparks. The spark current should be unidirectional to enhance the discharge stability and to avoid the vaporization of solid material from the anode.

25 Claims, 10 Drawing Figures \smuoows 282 PATENTEDJUN 1 1 I974 "swan 1 or 5' CU R RENT INJECTION RA 0! 0 FR 5Q. VOLTAGE SOURCE PATEN'TEDM 11 m4 3315995 SHEET H11 6 P'A'TEN-TEUJuuI 1 m4 SHEEI 5 OF 6 TIME I N MICROSECONDS METHOD AND APPARATUS FOR SPARK SPECTROSCOPY BY DERIVING LIGHT FROM LIMITED PORTIONS OF THE SPARK DISCHARGE The invention described herein was made in the course of or under a grant from the National Science Foundation, an agency of the United States Government.

This invention relates to spectroscopic analysis of sample material by emission spectroscopy of the light derived from a spark discharge across a spark gap between two electrodes, at least one of which is made of or supplied with the sample material so that the sample material is vaporized locally by the intense heat of the spark discharge.

In the copending application of John P. Walters, Ser. No. 66,920, filed Aug. 26, 2970, it has been disclosed that it is advantageous to adjust the wave form of the spark current across an analytical spark gap so that the spark current is uni-directional, or nearly so, between anode and cathode electrodes, at least one of which is made of or supplied with the sample material to be analyzed. The unidirectional spark current improves the signal-to-noise ratio of the analytical spectra so that the desirable spectral lines due to constituents of the sample material are more prominent relative to the background spectral lines.

The copending application of John P. Walters and Thomas V. Bruhns, Ser. No. 8,462, filed Feb. 4, 1970, now US. Pat. No. 3,653,766 issued Apr. 4, 1972 has disclosed that it is advantageous to ignite repetitive analytical spark discharges by a pulsed radio frequency voltage source connected to the spark gap by a resonant line which steps up the radio frequency voltage so that it is sufficiently high to break down the spark gap. The resonant line is preferably arranged so that current of a desirable wave form may be injected into the spark gap by way of a nodal point along the line. Such injected current is effective to vaporize sample material from the cathode electrode of the spark gap.

One object of the present invention is to provide a new and improved method and apparatus which may utilize the disclosures of the above-mentioned prior applications, but which utilizes additional improvements whereby greatly improved results are achieved by making spectral analyses of sample materials. Such improvements include great improvements in the signalto-noise ratio so that the desired spectral lines due to constituents of the sample material are much more prominent relative to background spectral lines and interferences. Moreover, the present invention achieves much greater precision and reproducibility in the results of spectral analyses than has been possible heretofore.

In accordance with the present invention, it is preferred to stabilize the spark discharge so that each spark always follows substantially the same path or channel between the electrodes of the spark gap. Thus, the spark discharge stands still in that successive sparks of a repetitive discharge all follow substantially the same channel rather than dancing about or jittering. The stabilized spark discharge is then employed for analytical spectroscopy by deriving the light for such spectroscopy from the luminous wings of the spark discharge, outside the defined spark channel.

A mask may be employed to block out the light from the spark channel, or the light for the spectroscopic analysis may be derived from a window which includes the wings of the spark discharge, or a portion of such wings, but not any substantial portion of the spark channel. It has been found that the light derived from the wings of the spark discharge is relatively rich is spectral lines due to constituents of the sample material, while being relatively free from background lines and interference due to the ionized gas in the spark channel and also due to interaction between the ionized gas and the sample vapor in the spark channel. The spark discharge causes explosive vaporization of the sample material from the surface of the cathode in the spark channel. The sample vapor expands rapidly to form the wings of the discharge outside the spark channel.

It is advantageous to roughen the surface of the sample electrode or otherwise to form peaks or points between so that the sample material is vaporized from such peaks or points rather than from valleys and thus is thrown outwardly from the spark channel into the wings of the discharge rather than being thrown into the spark channel. It is advantageous to produce relative movement between the sample electrode and the spark channel, either continuously or intermittently, so that the spark discharge does not get a chance to erode a deep pit or hole in the electrode surface. When such a pit has been formed, the vapor of the sample material tends to be thrown into the spark channel rather than outwardly into the wings of the discharge.

The spark discharge may be stabilized by producing the discharge between a sharply pointed anode and the sample cathode, while surrounding the pointed anode by a laminar stream of argon, or some other readily ionizable gas, which flows across the spark gap parallel to the axis thereof and impinges upon the cathode. The sharply pointed anode is preferably mounted axially in a nozzle opening which directs the gas flow so that the axis of the stream of gas coincides with the axial line of maximum electrical field strength between the sharply pointed anode and the cathode. The spark channel then forms along such line. The gas flow is at a sufficiently high rate to form an uninterrupted stream between the anode and the cathode and to sweep away or spatially arrange the produces of the spark discharge between the repetitive sparks.

It is often advantageous to derive the light for spectral analysis from a particular time segment following the ignition of the spark discharge to take advantage of differences between the velocities at which different constituent vapors are propagated from the sampling site. Due to such velocity differences, the spectral line due to a particular constituent may be more prominent, relative to other spectral lines, in a particular time segment than in other segments.

Further objects, advantages and features of the present invention will appear from the following description, taken with the accompanying drawings, in which:

FIG. 1 is a diagrammatic sectional view of apparatus for producing a stabilized spark discharge in accordance with the present invention.

FIG. 2 is a fragmentary sectional view similar to the upper portion of FIG. 1, but showing a modified construction whereby the sample electrode may be rotated.

FIG. 3 is a view similar to FIG. 2, but showing another modified construction for rotating the sample electrode.

FIG. 4 is a diagrammatic view on an enlarged scale of the spark gap for producing the stabilized spark discharge.

FIG. 5 comprises a set of graphs showing the discharge current and the intensity of a particular spectral line for a stabilized spark discharge.

FIG. 6 is a series of diagrams showing the two systems of a stabilized spark discharge comprising the spark channel and the expanding sample vapor which forms the wings of the discharge.

FIG. 7 comprises oscillograms and a series of illustrations showing successive stages of an actual spark discharge in which the spark channel and the expanding vapor are visible.

FIG. 8 comprises a series of profile graphs showing the variation in the intensity of a particular spectral line as a function of the radial distance from the center of the spark channel, the distance from the surface of the sample, and the elapsed time for the ignition of the spark discharge.

FIG. 9 comprises a series of graphs showing the variation in the intensity of a particular spectral line as a function of time for various distances from the sample surface, and for two different discharge current wave forms.

FIG. 10 is a diagram illustrating the spark discharge and various methods of deriving light from the discharge so as to produce improved analytical results.

The advantages of the present invention can be achieved to the greatest possible extent by producing a stabilized spark discharge to serve as the light source for performing spectroscopic analyses. FIGS. 1-4 illustrate apparatus for producing such a spark discharge which is stabilized so that the spark stands still, in that the successive sparks of a repetitive spark discharge follow the same path or channel. It will be seen that FIG. 1 illustrates an analytical spark discharge for producing such a stabilized spark discharge.

The spark gap 20 comprises a sample electrode 22, usually serving as the cathode, and a second electrode 24, normally acting as the anode. As shown, the sample electrode 22 simply takes the form of a cylindrical rod having a flat end surface 26, which confronts the other electrode 24. The illustrated end surface 26 is perpendicular to the cylindrical sample electrode 22.

The anode electrode 24 is preferably in the form of a cylindrical pin having a sharp point 28 directed toward the flat end surface 26 of the sample electrode 22 along a line or axis which is perpendicular to such surface. When a voltage is produced between the electrodes 22 and 24, the line of maximum electric field intensity extends between the electrodes along the axis of the sharply pointed electrode 24. This electrode construction contributes to the stabilization of the spark discharge.

Another contributing factor is provided by a stream of argon, or some other readily ionizable gas, which is preferably directed so as to surround the sharply pointed pin 24. The stream of gas is directed along the pin 24 with sufficient velocity to flow across the spark gap and to impinge upon the flat end surface 26 of the sample electrode 22. The rate of gas flow should be sufficiently rapid to prevent any significant slumping of the gas stream and to maintain the direction and continuity of the stream so that its flow lines are substantially parallel to the axis of the spark gap. At the same time, it is important to maintain laminar flow of the gas. The flow rate should not be so great as to cause turbulence.

It should be noted that the axis of the laminar stream of gas coincides with the line of maximum electric field intensity between the sharply pointed pin 24 and the end surface 26 of the sample electrode 22. When a spark is ignited under these conditions by providing sufficient voltage between the sharply pointed pin 24 and the electrode 22, the spark will follow a welldefined channel and will not jiggle or dance laterally relative to the axis of the spark gap. Argon is the preferred gas to produce a stabilized spark discharge, but other readily ionizable gases, such as neon, for example, can also employed.

In the apparatus of FIG. 1, the argon gas is directed along the sharply pointed anode pin 24 by a nozzle or tube 30 having an axial opening 32 therein. It will be seen that the pin 24 is positioned in the opening 32 along the axis thereof. The size of the opening 32 is substantially greater than that of the pin 24 to produce an annular space 34 between the pin and the opening 32 in the nozzle 30. The pin 24 preferably projects some distance beyond the end of the nozzle 30. The argon gas is suitably supplied to the opening 32 within the nozzle 30, as by means of a side tube 36. A hose or other conduit 38 is connected between the side tube 36 and a pressure tank or some other source 40 of the argon gas.

FIG. 4 illustrates diagrammatically typical flow lines of the stream of argon gas 42 which surrounds the anode pin 24 and is directed toward the sample electrode'22 so as to impinge upon the flat end surface 26 thereof. It is important to maintain laminar flow in the argon stream. Thus, the rate of flow should not be so great as to cause turbulence. However, the flow rate should be sufficiently great to maintain the continuity and the parallel lines of the argon stream between the anode pin 24 and the sample electrode 22.

As shown in FIG. 1, the cylindrical sample electrode 22 is mounted in a collet-type clamp 44 which is tightened by turning a hand wheel 46. When the clamp is loosened, the sample electrode 22 may be inserted, adjusted or removed. A high voltage source is required to produce sparks across the spark gap 20.

In the apparatus of FIG. 1, the high voltage is produced by a radio frequency voltage source 48 connected to the spark gap 20 by a quarter wave resonant line 50, as disclosed in the previously-mentioned copending application of John P. Walters and Thomas V. Bruhns, Ser. No. 3,462, filed Feb. 4, 1970. The resonant line 50 acts as a voltage increasing device to produce a high voltage for spark ignition when supplied with radio frequency current at a lower voltage by the source 48. To produce repetitive sparks, the radio frequency source 48 is repetitively pulsed.

As disclosed in the above-mentioned Walters and Bruhns application, the high voltage source, involving the resonant line 50, makes it possible to inject current of various wave forms into the spark discharge when it has been ignited by the high radio frequency voltage. Thus, a current injection source 52 is connected to the resonant line 50, preferably at a nodal point therealong, at which there is very little radio frequency voltage. For example, the current injection source 52 may comprise a charged capacitor which is discharged across the spark gap when the spark is ignited by the radio frequency voltage.

The wave form of the injected current may be controlled in the manner disclosed and claimed in the co pending application of John P. Walters, Ser. No. 66,920, filed Aug. 26, 1970. It is generally preferable to control the wave form so that the injected current is uni-directional, or nearly so, so that material will be vaporized only from the sample electrode 22 and not from the pin electrode 24. Moreover, the discharge will not be stable unless the current is uni-directional.

As shown, the resonant transmission line 50 comprises an axial, generally cylindrical inner conductor 54 and a coaxial, generally cylindrical outer conductor 56. The inner conductor 54 is connected to the nozzle tube 30 and thence to the anode pin 24 by way of a bushing or plug 58 in which the pin is clamped or otherwise mounted. By means of the collet clamp 44, the sample electrode 22 is connected to a conductive end wall 60, which in turn is connected to the outer conductor 56 of the resonant line 50. It will be seen that the radio frequency source 48 is connected between the inner and outer conductors 54 and 56 of the resonant line 50, at the end thereof remote from the spark gap 20. The current injection source 52 is connected between the inner and outer conductors 54 and 56 at an intermediate point along the line 50.

With the arrangement of FIG. 1, the spark discharge can be effectively stabilized so that the sparks always follow substantially the same path or channel, even at repetition rates, running well in excess of 1,000 sparks per second.-As the repetition rate is increased, it is eventually necessary to increase the rate of gas flow in order to maintain the stability of the discharge. The upper limit of the repetition rate for producing a stable spark discharge is reached when the rate of gas flow becomes so high as to cause substantial turbulence in the stream of gas. Such turbulence will cause the spark discharge to become unstable.

The required rate of gas flow is related to the spark repetition rate in that the rate of gas flow should be sufficient to sweep away the products of each spark in the time interval between the successive sparks. If ionized gases are left over in the vicinity of the electrodes between the successive sparks, the position of the spark channel may not always be the same so that instability will be observed in the spark discharge.

Each spark generates intense heat which vaporizes a small amount of the sample material from the surface of the sample electrode 22. To prevent the successive spark discharges from eroding a definite pit or hole in the sample, it is desirable to produce relative movement between the sample and the spark channel so that new elements of the sample will progressively be exposed to the sampling action of the successive sparks.

FIGS. 2 and 3 show modified arrangements for causing such movement. In each case, the sample is rotated about an axis which is offset from the axis of the spark discharge. With the passage of time, the sample is rotated either intermittently or continuously by manual or power-driven means. In FIG. 2, the sample electrode is in the form of a cylindrical disc or slug 122 having a flat end surface 126 end surface 126 confronting the sharply pointed anode 24. By means of a suitable holder, the sample electrode 122 is connected to a disc or other member which can be rotated manually. Such rotation is about an axis which is displaced from the axis of the spark gap so that the surface 126 will be moved laterally relative to the spark channel when the sample electrode is rotated.

In FIG. 2, a locking or indexing device 132 is provided to hold the rotatable member 130 in any of several positions. As shown, the locking device 132 comprises a manually operable pin [34 which may be moved into or out of any of a series of angularly spaced openings 136 in the rotatable member 130.

In FIG. 3, the sample electrode is in the form of a disc or slug 222 having a flat surface 226 confronting the sharply pointed anode pin 24. The sample disc 222 is suitably mounted on a holder 230 which may be rotated by means of a motor 232. A suitable driver 234 is provided between the motor 232 and the holder 230. The motor 232 may produce continuous rotation or may be of the stepper type to produce intermittent rotation. In the latter case, the stepping of the motor may be synchronized with the repetitive spark discharge current.

The movement of the sample during the series of repetitive spark discharges imrpoves the single-to-noise ratio and resolution achieved in the spectroscopic examination of the light from the spark discharges. This point will be elaborated presently.

The stabilization of the spark discharge results in a corresponding stabilization of the light output from the discharge. It has been found that the spectra derived from the stabilized spark discharge are stable and reproducible to a much greater extent than heretofore. This observation is illustrated in FIG. 5, which is a set of oscillograms showing the wave from of the spark discharge current and the intensity of a copper spectral line for a stabilized spark discharge. The intensity of the spectral line is plotted in relative units as a function of the elapsed time from the initiation of each spark. FIG. 5 represents the superimposed oscillograms for many thousands of repetitive sparks. It will be evident that the oscillograms for the discharge current and the spectral line intensity show a very high degree of reproducibility with very little deviation of the individual oscillograms from the overall pattern. The detailed conditions under which these oscillograms were made are as follows:

Sample Rotation Speed V4 rpm Argon Flow Rate 0.4 ltlmin Electrode Separation 2.5 mm Observation Window Width 0.1 mm

Observation Window Position 0.2 mm from cathode surface Monochromator Slits 0.05 mm Photomultiplier Voltage H00 volts Photomultiplier Load 50 ohms Sparking Frequency 2900/sec No. of Sparks/Time Sweep 17400 No. of Sweeps Superimposed 5 The stabilization of the spark discharge makes it possible to examine all of the different portions of the spark discharge in minute detail. As illustrated diagrammatically in FIG. 6, the stabilized spark discharge comprises two systems, the spark channel 240 and an expanding cloud or plume 242 of material vaporized from the sample electrode 22. It wild be understood that this statement and all other subsequent statments arealso applicable to the sample electrodes 122 and 222.

The spark discharge generates intense heat on the surface of the sample within the spark channel 240. Such heat causes explosive vaporization of material from the sample. The vaporized material produces the rapidly expanding vapor plume 242 which is substantially hemispherical in shape when the surface of the sample is flat or approximately so.

The successive diagrams, designated a-f in FIG. 6, illustrate the progressive expansion of the vapor plume 242. It will be seen that a portion of the plume 242 is within the spark channel 240. However, the rapidly expanding plume 242 produces wings 244 which are outside the spark channel. These wings of the discharge are luminous and produce spectra which are characteristic of the various constituents of the sample electrode 22. It will be recognized that the wings 244 are actually connected segments of the luminous annular portion of the vapor plume 242 outside the spark channel 240.

As the spark current changes during each spark discharge, the width of the spark channel 240 also changes. As will be evident from FIG. 6, the width of the spark channel increases with increasing discharge current, and decreases with decreasing c'urrent. However, the vapor volume plume 242 continues to expand throughout the time interval of the spark discharge. The continuing discharge current causes additional material to be vaporized into the vapor plume 242 at the sampling site within the spark channel 240.

The spark channel 240 and the expanding vapor plume 242 are clearly visible in FIG. 7, which comprises drawings representing a series of actual photographs of a spark discharge taken at successive intervals after the ignition of the spark. The oscillograms shown in parts a and b of FIG. 7 illustrate the voltage wave form 250 and the current wave form 252 across the spark gap 20, which is shown in part c of FIG. 7. The position in FIG. 1. Thus, the pointed anode 24 is above, while the sample cathode 26 is below.

Parts d-g of FIG. 7 represent successive photographs of the spark discharge. The expanding width of the spark channel 240 with increasing discharge current is clearly visible. The formation of the vapor plume 242 can be seen in Part d within the brackets at the lower end of the spark channel. The progressive expansion of the vapor plume 240 is evident from Parts e, f and g of FIG. 7.

It has been found that greatly improved spectral analyses can be performed by excluding the light from the spark channel and deriving the light for the spectral analyses from the luminious wings of the spark discharge. The light from the wings is due very largely to the excited vapor exploded from the sample electrode by the spark discharge. Thus, the light from the wings of the discharge is rich in spectral lines corresponding to the various constituents of the sample material.

On the other hand, the light from the spark channel contains a heavy loading of spectral lines due to the ionized argon or other gas which supports the spark discharge. These spectral lines tend to obscure the spectral lines due to the constituents of the sample materials. Moreover, in the spark channel, the excited sample vapor interacts with the ionized argon or other gas-t produce additional complex spectral lines and other spectral elements which increase the background and tend to cause interference with the desired spectral elements due to the constituents of the sample vapor.

By excluding the light from the spark channel, and utilizing the light from the luminous wings of the discharge for spectral analysis, the spectral lines due to the various constituents of the sample material show up much more clearly with less background and greatly improved signal-to-noise ratio. Moreover, the intensity of the desired spectral lines due to the sample material is generally greater in the luminous wings of the .discharge than in the spark channel.

This situation is illustrated with great clarity in FIG. 8 in which the intensity of a single copper spectral line has been mapped as a function of the radial distance from the axis of the spark channel, the distance from the surface of the sample electrode, and the elapsed time from the initiation of the spark discharge current.

Twelve sets of maps are illustrated for successive time intervals, ranging from 0.5 microseconds to 6.0 microseconds from the beginning of the spark current. In each map, the radial distance from the center of the spark channel is plotted along the horizontal or X axis. The intensity of the spectral line is plotted along the vertical or Y axis.

The various graphs for different distances from the surface of the sample electrode are distributed along the Z axis at locations designated A-H, where location A is at the surface of the sample electrode and the other locations are at progressively increasing distances therefrom. These radial maps clearly show the movement of vaporized electrode material and also the full development of the wing structure in the fringes of the discharge. It will be seen that the luminous vapor from the sample electrode has been propagated only to the distance B in 0.5 microseconds, and that the luminous vapor does not reach location H until 3 microseconds after the beginning of the spark current.

The radial expansion of the luminous vapor cloud can also be seen in that the luminous wings extend outwardly to a radius of only about 0.5 mm to at 0.5 microseconds, while extending outwardly to about 1.75 mm after 6 microseconds.

It will be seen from FIG. 8 that the various graphs are characterized by peaks 260 of high intensity in the wings of the discharge. These peaks move out radially with the lapse of time due to the expansion of the luminous vapor cloud. In the center of the spark channel, the intensity of the copper line is less than in the wings of the discharge. It is thus clearly advantageous to restrict the viewing area to the wings of the discharge.

It will be evident from FIG. 8 that the intensity of any particular spectral line varies as a function of the elapsed time after the beginning of the spark discharge current. This variation is further illustrated in FIG. 9 in which the intensity of an aluminum spectral line known as Al I, which has wavelength of 3,082 angstroms, is plotted against the elapsed time from the beginning of the spark current.

FIG. 9 includes a series of graphs for different distances from the surface of the sample cathode, ranging from 0 to 0.7 mm. Two sets of graphs are shown for two different spark current wave forms. It will be evident that the wave form has a considerable effect upon the variation of the intensity. The wave forms are shown in Parts a and j of FIG. 9, while the various intensity graphs are shown in Parts b-i and k-r. In each case, the intensity tends to rise an initial peak 270. With the passage of time, the intensity then falls to a valley or minimum 272 and then tends to rise to another peak 274. Additional valleys and peaks are evident in some cases.

It will be evident that the time segment or segments employed in making spectral analyses must be controlled in order to obtain reproducible results. The wave form of the spark current also needs to be controlled. By preselecting a particular time segment and a particular distance from the sample, it is possible to utilize one or more of the peaks in the intensity curve. In this way, greatly improved spectra analyses are possible.

It will be evident from FIG. 9 that the outward velocity of the vaporized sample material is a factor in the variation of the line intensity with the passage of time. There is a definite shifting or increase in the time element as the distance from the sample cathode surface is increased. The velocity at which the sample vapor is propagated is an inverse function of the atomic mass of the material which is vaporized. Thus, the vapor of different constituents in the sample material will be propagated at different velocities. It is often advantageus to take advantage of this situation by selecting a particular time segment which makes it possible to take advantage of such velocity differences. For example, when the sample material is cast iron or steel, velocity differences can be utilized to produce improved spectral analyses which will show the percentage of carbon in relation to iron in the sample material. Carbon vapor is propagated much more rapidly than iron vapor because carbon has a much lower atomic mass.

In the spectra of cast iron and steel, the iron spectral lines tend to cause interference with the carbon spectral lines. This is particularly noticeable when the percentage of carbon is low so that the intensity of the carbon lines is correspondingly low. It is possibele to overcome such interference effects by utilizing a viewing window which is focused upon the outer wings or fringes of the spark discharge. The carbon vapor will be propagated to this window before the slower moving iron vapor arrives. By utilizing a time segment which begins after the arrival of the carbon vapor and ends with the arrival of the iron vapor, it is possible to make spectrograms in which the carbon lines are much more prominent, relative to the iron lines, than would be the case without such control of the time element.

FIG. 10 is similar to FIG. 6 in that it illustrates the spark channel 240 and the expanding vapor cloud or plume 242, which, however, is shown in terms of an expanding hemispherical shell corresponding to the material which is vaporized during any particular spark discharge. As will be evident, the shell expands outwardly to a series of successive positions. Outside the spark channel 240, the expanding shell produces the wings of the discharge.

As already indicated, improved spectral analyses can be produced by excluding the light from the spark channel 240 and utilizing only the light from the wings of the discharge. The light from the spark channel 240 can be excluded by using a mask 280 to cover the spark channel while leaving the wings of the discharge substantially unobscured. The mask 280 may be in the form of a wire or any other suitable member corresponding in width to the width of the spark channel. The mask may be somewhat wider, the same width or somewhat narrower than the spark channel. The mask 280 is also indicated in FIG. 6 in which the width of the mask corresponds to the maximum width of the spark channel 240.

It is also possible to employ one or more windows 282 which do not include the spark channel 240, but do include various portions of the luminous wings of the discharge. By way of example, windows 282 of various sizes and locations are shown in FIG. 10. The windows may be in the form of slits or other openings in any suitable member. In some cases, it will be desirable to employ a window located on or close to the spark channel and opposite the inner wings of the spark discharge. Such a window will tend to favor the spectral lines for constituents as to which the propagation of the vapor is relatively slow and of limited range. On the other hand, a window opposite the outer wings of the discharge will tend to favor a constituent as to which the vapor propagation is rapid and of greater range. Moving the window toward or away from the surface of the sample has somewhat the same effect. Where the total amount of light is to be maximized, one or more large windows, may be employed covering all or large portions of the wings of the discharge.

As already indicated, the time element may be resolved so that only a particular time segment is employed in making the spectral analyses. However, in many cases, the time element is integrated over the entire time interval of the spark discharge. With the stabilization of the spark discharge, quite a large number of repetitive sparks may be employed in making any particular spectral analysis. In this way, a sufficient amount of light is integrated to achieve highly reliable results.

In FIGS. 1-4, the sample electrode 22 is shown with a flat surface 26 from which the sample material is vaporized by the spark discharge. The modified sample electrodes 122 and 222 also have flat surfaces 126 and 226. This construction is advantageous in that the provision of the flat surface contributes to the stabilization of the spark discharge. The flat surface is perpendicular to the axis of the sharply pointed anode pin 24. Thus, the line of maximum electric field intensity coincides with such axis.

However, the active surface of the sample electrode may assume various shapes other than flat. Thus, the active surface of the sample electrode may be rounded or pointed, for example. The active surface of the sample electrode is preferably somewhat rough rather than being smoothly polished. In preparing the sample electrode, it is preferably to roughen the active surface which is to form one side of the spark gap. The surface may be roughened with sandpaper or other abrasive material. Chemical etching may also be employed to roughen the sample surface.

The roughening of the active sample surface produces a multiplicity of peaks or points on the surface. When the spark discharges are produced between the anode and the sample electrode, the spark discharges cause vaporization of the sample material from the peaks or points on the active sample surface rather than from the valleys.

As previously indicated, the sample material is vaporized explosively by the heat of the spark discharges. When the material is vaporized from the peaks, the vapor is propagated outwardly without any obstruction. Thus, the vapor cloud is hemispherical in shape. If the material is vaporized from valleys in the sample surface, the outward propagation of the vapor is obstructed by the adjacent higher elements on the surface of the sample. Thus, more of the vapor is thrown into the spark channel. The light output from the sample vapor in the spark channel is of diminished value for spectral analysis due to the increased background caused by the spectral lines from the ionized gas in the spark channel.

As already indicated in connection with FIGS. 2 and 3, it is advantageous to produce relative lateral movement between the sample electrode and the spark channel so that new surface elements of the sample electrode are constantly being presented to the spark discharge. In this way, the spark discharge does not get a chance to erode a deep pit or hole in the surface of the sample. By suitably moving the sample, the erosion due the spark discharges can be distributed over the active surface of the sample electrode.

In the arrangements of FIGS. 2 and 3, the relative movement between the sample electrode and the spark channel is produced by rotating the electrode about an axis which is displaced from the axis of the sharply pointed anode pin 24. Such rotation produces lateral movement of the sample along the plane of its flat active surface.

In the arrangement of FIG. 2,the sample electrode 122 may be rotated by manually turning the disc or head 130. In FIG. 3, the rotation of the sample 222 is produced by the motor 232. The motor may cause the sample electrode to be rotated continuously or step by step. The pulses employed to trigger the spark discharges may also be employed to control the stepwise operation of the motor.

The formation of the sample electrode with a flat active surface has the advantage that this construction makes it readily possible to move the sample electrode laterally without changing the distance between the electrodes of the spark gap. Inasmuch as the movement of the electrode prevents the spark channel from eroding a pit in the active surface of the sample, the sample vapor is propagated outwardly without the obstruction that would be caused by the walls of any such pit. Accordingly, the proportionate amount of the luminous sample vapor in the wings of the discharge is increased so that improved spectral analyses can be made based on the light derived from such wings.

As previously indicated, the active surface of the sample electrode may be of various shapes other than flat. However, the surface should preferably be symmetrical about the axis ofthe sharply pointed anode 24. In this way, the line of maximum electric field strength will coincide with the axis of the sharply pointed anode.

The anode 24 is generally made of a material which is highly resistant to heat. Thus, tungsten and carbon have been employed successfully. Thoriated tungsten is also an advantageous material.

The sample electrode is made or of supplied with the sample material which is to be analyzed. Virtually any material may be analyzed, but spectroscopic analyses are particularly valuable for metals and metal alloys. By making spectroscopic analyses, it is possible to determine very quickly the amount of each constituent in a metal or metal alloy, and also the amount of any impurities that may be present.

The surface of the sample may be roughened chemically or by any mechanical roughe'ning method. Quite often, the sample electrode is cut or shaped by mechanical methods, such as machining or abrasion, which leave the surface of the sample in a sufficiently roughened condition without any additional processing.

We claim: 1. A spectroscopic method, comprising the steps of producing a spark discharge having a spark channel between a sample electrode and a second electrode, deriving light from a luminous region of said discharge spatially disposed outside said channel but not inside said channel to any substantial extent. and utilizing said light for spectroscopy. 2. A method according to claim 1, in which the light from said channel is excluded by masking said channel.

3. A method according to claim 1,

in which said light is derived from utilizing a window confronting a luminous region of said discharge outside said channel but not including said channel.

4. A method according to claim 1, I

in which the size and location of said region are preselected to afford improved and reproducible results from said spectroscopy.

5. A method according to claim 1,

in which said spectroscopy is carried out in a preselected time segment relative to the beginning of the spark discharge.

6. A method according to claim 1,

including the additional step of producing relative lateral movement between said sample electrode and said spark channel to prevent the spark discharge from eroding a deep pit in said sample electrode.

7. A method according to claim 1,

including the additional step of rotating said sample electrode to produce lateral movement between said electrode and said spark channel whereby the erosion of a pit in said sample electrode by said spark discharge is prevented.

8. A method according to claim 1,

including the additional step of providing surface roughness on said sample electrode so that material vaporized from said electrode by said spark discharge will be vaporized from the higher surface elements on said electrode.

9. A method according to claim 8,

in which said surface roughness is produced by mechanically roughening the sample electrode.

10. A method according to claim 8,

in which said surface roughness is produced by abrasion of said sample electrode.

11. A method according to claim 8,

in which said surface roughness is produced by chemical action upon said sample electrode.

12. A method according to claim 1,

in which said spark discharge is stabilized by providing a laminar stream of a readily ionizable gas between said electrodes.

13. A method according to claim 1,

in which said spark discharge is stabilized by producing a laminar stream of argon between sid electrodes.

14. A method according to claim 1,

in which said spark discharge is stabilized by providing a laminar stream of a readily ionizable gas between said electrodes,

and by producing an electrostatic field between said electrodes and having a line of maximum field intensity corresponding generally to the axis of said laminar stream.

15. A method according to claim 14,

in which said readily ionizable gas is argon.

16. A method according to claim 1,

in which said spark discharge is stabilized by producing a laminar stream of readily ionizable gas between said electrodes,

said spark discharge being repetitive and thus com prising a series of repetitive sparks,

the repetition rate of the sparks being related to the rate of flow of the ionizable gas such that the products of each spark are swept away from the electrodes during the intervals between the repetitive sparks.

17. A method according to claim 1,

in which said light is derived from a time segment related to the location of said region such as to allow for the velocity at a constituent of the sample vapor is propagated to said region after being vaporized from the sample electrode by the spark discharge.

20. Apparatus according to claim 18,

in which said selective means includes a window element confronting the luminous wings but not the spark channel of said spark discharge.

21. Apparatus according to claim 18,

in which at least one of said electrodes includes sample material,

said apparatus including means for producing relative lateral movement between said sample material and said spark channel to prevent the erosion of a deep pit in said sample material by the spark discharge.

22. Apparatus according to claim 18,

in which at least one of said electrodes includes sample material,

said apparatus including means for causing rotation between said sample material and said spark channel about an axis displaced from said spark channel.

23. A spectroscopic method,

comprising the steps of producing a spark discharge between a sample electrode and a second electrode and having a spark channel and luminous wings spatially disposed outside said channel,

deriving light from said luminous wings of said spark discharge but not from said spark channel,

and utilizing said light for spectroscopy.

24. A method according to claim 23,

in which said light is derived exclusively from a portion of said luminous wings remote from said channel.

25. A method according to claim 23,

in which said light is derived exclusively from a portion of said luminous wings close to said spark channel.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,815,995 Dated June 11, 1974 Inventor) Walters et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 1, lines 46 and 47 "improvements" should be improved. results Col. 2, line 53, "line" should be lines Col. 4, line 53, "Ser. No. 3462 should be Ser. No. 8462 Col. 5, line 32, before "repetition" add high Col. 6, line 64, "wild" should be will Col. 6', line 65, "statments" should be statements Col. 6, line 66, "arealso" should be are also Col. 7, line 38, "The position in Fig. 1'" should read: The position of the spark gap is inverted, relative to the position shown in Fig. L

Col. 9, line 36, "possibele" should be possible In The Claims Claim 13, Col. 12 "sid" should be said Claim 17, Col. 13, Q line 21 after "at" insert which Signed and sealed this 1st day of April 1975.

(SEAL) Attest:

C. I- TARSHALL DANN RUTH C. MASON Commissioner of Patents attesting Oficer and Trademarks 

1. A spectroscopic method, comprising the steps of producing a spark discharge having a spark channel between a sample electrode and a second electrode, deriving light from a luminous region of said discharge spatially disposed outside said channel but not inside said channel to any substantial extent. and utilizing said light for spectroscopy.
 2. A method according to claim 1, in which the light from said channel is excluded by masking said channel.
 3. A method according to claim 1, in which said light is derived from utilizing a window confronting a luminous region of said discharge outside said channel but not including said channel.
 4. A method according to claim 1, in which the size and location of said region are preselected to afford improved and reproducible results from said spectroscopy.
 5. A method according to claim 1, in which said spectroscopy is carried out in a preselected time segment relative to the beginning of the spark discharge.
 6. A method according to claim 1, including the additional step of producing relative lateral movement between said sample electrode and said spark channel to prevent the spark discharge from eroding a deep pit in said sample electrode.
 7. A method according to claim 1, including the additional step of rotating said sample electrode to produce lateral movement between said electrode and said spark channel whereby the erosion of a pit in said sample electrode by said spark discharge is prevented.
 8. A method according to claim 1, including the additional step of providing surface roughness on said sample electrode so that material vaporized from said electrode by said spark discharge will be vaporized from the higher surface elements on said electrode.
 9. A method according to claim 8, in which said surface roughness is produced by mechanically roughening the sample electrode.
 10. A method according to claim 8, in which said surface roughness is produced by abrasion of said sample electrode.
 11. A method according to claim 8, in which said surface roughness is produced by chemical action upon said sample electrode.
 12. A method according to claim 1, in which said spark discharge is stabilized by providing a laminar stream of a readily ionizable gas between said electrodes.
 13. A method according to claim 1, in which said spark discharge is stabilized by producing a laminar stream of argon between sid electrodes.
 14. A method according to claim 1, in which said spark discharge is stabilized by providing a laminar stream of a readily ionizable gas between said electrodes, and by producing an electrostatic field between said electrodes and having a line of maximum field intensity corresponding generally to the axis of said laminar stream.
 15. A method according to claim 14, in which said readily ionizable gas is argon.
 16. A method according to claim 1, in which said spark discharge is stabilized by producing a laminar stream of readily ionizable gas between said electrodes, said spark discharge being repetitive and thus comprising a series of repetitive sparks, the repetition rate of the sparks being related to the rate of flow of the ionizable gas Such that the products of each spark are swept away from the electrodes during the intervals between the repetitive sparks.
 17. A method according to claim 1, in which said light is derived from a time segment related to the location of said region such as to allow for the velocity at a constituent of the sample vapor is propagated to said region after being vaporized from the sample electrode by the spark discharge.
 18. Spectroscopic apparatus, comprising first and second electrodes forming a spark gap, means for producing a spark discharge extending across said gap and having a spark channel, siad spark discharge having luminous wings spatially disposed outside said channel, and selective means for deriving light for spectroscopic analysis from said wings but not from said spark channel.
 19. Apparatus according to claim 18, in which said selective means includes a mask for obscuring said spark channel.
 20. Apparatus according to claim 18, in which said selective means includes a window element confronting the luminous wings but not the spark channel of said spark discharge.
 21. Apparatus according to claim 18, in which at least one of said electrodes includes sample material, said apparatus including means for producing relative lateral movement between said sample material and said spark channel to prevent the erosion of a deep pit in said sample material by the spark discharge.
 22. Apparatus according to claim 18, in which at least one of said electrodes includes sample material, said apparatus including means for causing rotation between said sample material and said spark channel about an axis displaced from said spark channel.
 23. A spectroscopic method, comprising the steps of producing a spark discharge between a sample electrode and a second electrode and having a spark channel and luminous wings spatially disposed outside said channel, deriving light from said luminous wings of said spark discharge but not from said spark channel, and utilizing said light for spectroscopy.
 24. A method according to claim 23, in which said light is derived exclusively from a portion of said luminous wings remote from said channel.
 25. A method according to claim 23, in which said light is derived exclusively from a portion of said luminous wings close to said spark channel. 